The protocol commonly used for secure data transmission over the internet is HTTPS, which stands for Hypertext Transfer Protocol Secure. It incorporates SSL/TLS encryption to protect data while in transit between a user's browser and a website. This protocol not only enhances security but also builds trust between the user and the site, as evidenced by the padlock icon often seen in browsers. HTTPS is essential for protecting sensitive information like passwords and credit card numbers, especially during online transactions.